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Power electronic control in electrical systems 107

only capable of providing passive compensation since their generation/absorption
of reactive power depends on their rating, and the voltage level at the connection
point. On the other hand, the reactive power generated/absorbed by synchronous
condensers and SVCs is automatically adjusted in order to maintain fixed voltage
magnitude at the connection points.
3. Load-tap changing transformers (LTCs), which are used to regulate voltage
magnitude at the LTC terminals by adjusting its transformation ratio.
If no control action is taken, active and reactive power flows in AC transmission
networks are determined by the topology of the network, the nodal voltage magni-
tudes and phase angles and the impedances of the various plant components making
up the network. However, stable operation of the power network under a wide range
of operating conditions requires good control of power flows network-wide. For
instance, reactive power flows are minimized as much as possible in order to reduce
network transmission losses and to maintain a uniform voltage profile. Reactive
power flow control may be achieved by generating/absorbing reactive power at
suitable locations in the network using one or more of the plant components men-
tioned above. On the other hand, the options for controlling the path of active power
flows in AC transmission networks have been very limited, with on-load phase
shifting transformers having provided the only practical option. These transformers
are fitted with a tap changing mechanism, the purpose of which is to control the
voltage phase angle difference across its terminals and, hence, to regulate the amount
of active power that flows through the transformer.

4.2 FACTS equipment representation in power flows

Until very recently, with the exception of the SVC, all plant components used in high-
voltage transmission to provide voltage and power flow control were equipment
based on electro-mechanical technology, which severely impaired the effectiveness
of the intended control actions, particularly during fast changing operating condi-
tions (Ledu et al., 1992). This situation has begun to change; building on the
operational experience afforded by the many SVC installations and breakthroughs
in power electronics valves and their control, a vast array of new power electronics-
based controllers has been developed. Controllers used in high-voltage transmission
are grouped under the heading of FACTS (Hingorani, 1993) and those used in low-
voltage distribution under the heading of Custom Power (Hingorani, 1995). The
most prominent equipment and their main steady state characteristics relevant for
power flow modelling are discussed below.

4.2.1 The SVC

From the operational point of view, the SVC behaves like a shunt-connected variable
reactance, which either generates or absorbs reactive power in order to regulate the
voltage magnitude at the point of connection to the AC network (Miller, 1982). In its
simplest form, the SVC consists of a TCR in parallel with a bank of capacitors. The
thyristor's firing angle control enables the SVC to have an almost instantaneous